And it is one of the scariest things you can imagine happening when you get in the water. Second only to sharks. Dozens of swimmers were attacked by piranha in a vicious feeding frenzy on christmas day. And matt gutman is back with this shocking story. Reporter: The only thing more frightening than a piranha in water is thousands of them. On christmas day, hundreds of beachgoers in the argentine city of rosario, scrambled from the river bank, as piranha attacked 60 of them. Biting off a young girl's fingers. And chomping into these men. This young boy says I came up there and they put a bannage on me. No one was killed in the attack. Dozens required medical aid. What's being called the worst attack there in memory. This paramedic said it was significant number of people. Some with injuries to their fingers. Another with a missing toe. And the rest were bite marks. A withering heat wave is being blamed for attracting the piranha to shore and the humans to the beach. In feeding frenzies like this one, devouring this white bird that fell into the amazon, piranha can devour everything, even bone. Piranha are freshwater fish with big teeth that feed into nightmares. Fodder for websites like piranha3d. A piranha bite is more powerful than great white sharks. More powerful than a tyrannosaurus rex. We learned when we swam with an canadas in the amazon last summer and came face with some of the cousins, the critters most often leave humans alone. But back in argentina, the bathers returned, hoping this time, they were the only species in the water. For "good morning america," matt gutman, abc news, miami. So scary. It has been reported that the fish may have been attracted to the coast left by debris from local fishermen. Terrifying, to be sure.

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